Cranes are in widespread industrial and commercial use throughout the world for lifting heavy objects. In many applications, it is not uncommon to subject cranes to overload conditions. An overloaded condition may develop relatively slowly over time, or it may occur suddenly and erratically. For example, if a pedestal crane at an offshore oil drilling station is used to unload heavy objects from a floating ship onto a platform of the offshore drilling station, the weight of a lifted object supported by the ship may be rapidly shifted to the crane as a result of sudden vertical movements of the ship during high sea conditions. The impact resulting from such a rapid load transfer may exceed the capacity of the crane and cause damage to either the crane or the lifted object. If the overload on the crane is severe, it may result in the collapse of the crane boom or in injury to the crane operator.
A more slowly developing crane overload condition may result from attempting to lift a heavy load with the boom at a shallow angle to the horizon. Since the load on the crane is a function of the moment or torque produced on the crane by the lifted object, the crane operator may readily reduce such a crane overload by increasing the angle of the boom to shorten the horizontal distance between the lifted object and the pivotal axis of the boom at the base of the crane.
There have been several attempts in the prior art to provide overload detection systems to protect cranes from overload conditions. In the simplest of these systems, a signal is generated in response to a detected overload to merely alert the crane operator. The operator must then take remedial action to reduce the load. Other systems simply interrupt the hoisting system in response to detected overload conditions.
The above described systems are incapable of responding to sudden, erratic and severe overload conditions. In recognition of such shortcomings, an overload protection system with a much more drastic and automatic response is disclosed in U.S. Pat. No. 4,107,798 to Comyns-Carr. In the Comyns-Carr system, the hoist cable of a crane is automatically paid out in reponse to a predetermined overload condition. The hoist cable is connected to the hoist drum by a tail cable of low tensile strength, which tail cable breaks under overload conditions to permit complete separation of the hoist cable from the hoist and boom. Releasing the hoist cable from the boom in this manner satisfactorily protects the crane from damage. However, such drastic action also permits the heavy object which caused the overload condition to drop uncontrollably. As a result, the object may either be damaged or lost in the sea. Furthermore, the dropped object may further damage the ship or cause injury to persons on the ship.
The optimum response to a particular overload condition is in large part dependent upon both the severity of the overload and the speed with which the overload condition is developing. Many of the less severe or slowly developing overload situations are optimumly corrected by remedial action on the part of the crane operator. If alerted, the operator can eliminate many of these predictable types of overload situations with simple remedial action, without any adverse consequences whatsoever. Other overload situations, however, are so unpredictable and develop with such speed that it is more desirable to automatically respond with a predetermined action initiated by a control system independently of operator action. Even for slowly developing and predictable overload conditions it is desirable for the control system to automatically intervene in the event that the operator does not take timely or appropriate remedial action.
The overload protection systems of the prior art have not adaptively distinguished between different types of overload situations and have provided only a single response to any overload exceeding a predetermined minimum threshold magnitude. It is generally desirable to avoid over responding to a crane overload condition and to react to an overload condition with the least severe responsive action necessary to adequately correct the problem. Nevertheless, it is desirable to have some provision for protecting the crane from severe overloads. More particularly, it is desirable to release the hoist cable from the hoist drum and to permit the hoist cable to pay out freely under extreme overload conditions. Given the limitations of the overload protection systems of the prior art, users have been relegated to balancing the adverse consequences of over-responding upon overload conditions to the dangers of under responding.